In drilling tools the demands on the periphery and on the center are different with respect to wear resistance and toughness. In drill bits for rock drilling the demands differ between the surface (wear resistance) and the inner part (toughness) as discussed in U.S. Pat. No. 5,541,006, in which is emphasized the use of two grades in a rock drilling bit. The grades are both straight grades with tungsten carbide and Co. Much attention is given to the ability to control the Co migration for which, in this case, an abrupt or discrete change of composition at the interface between the regions is preferred. This problem is also solved by Fischer with the technique known as Dual-Phase or DP-technique, U.S. Pat. No. 4,743,515. Tools as wear parts, rolling rings and slitter/trimming knifes can be manufactured with a method described in U.S. Pat. No. 5,543,235.
These patents, though, deal with combinations of grades containing only WC--Co or WC--Ni. They also refer to applications where just one of the grades is in contact with the work piece material, and the other serves as an `equalizer` or carrier` of pressure or impact.
One patent dealing with cemented carbide drills containing cubic carbides is U.S. Pat. No. 4,971,485, but in that case the WC--Co grade is used in the shaft to avoid damage due to vibrations emanating from the machine.
The present invention relates to a compound cemented carbide body consisting of a core of a tough grade and a surrounding tube of a more wear resistant grade that are both in active contact with the work piece material. The problem when making such a compound body is to avoid the formation of cracks in the outer part or voids and significant porosity at the interface between the two parts due to differences in shrinkage during sintering. In addition, too high stresses in the interface make further manufacturing, e.g., slitting and grinding, impossible. Another problem can be the migration of the binder phase during sintering which results in a leveling of the binder phase content in the two parts. The combination of grades has to fulfil the demands on toughness and wear resistance in the center as well as in the periphery. The grades also have to be compatible with respect to pressing conditions and sintering conditions.